Polymers having polyisocyanurate structure are known for their high thermal stability and flame resistance. Polyisocyanurate foams based on aromatic diphenylmethane 4,4′-diisocyanate (MDI) are in widespread use, particularly as high-performance insulating materials, for example because of their very low thermal conductivity. However, MDI polyisocyanurates, as is commonly known from aromatic polyurethanes, have only low light stability and have a tendency to significant yellowing.
There has therefore been no lack of attempts to synthesize polyisocyanurate plastics based on aliphatic light-resistant isocyanates.
For example, European Polymer Journal, Vol. 16, 147-148 (1980) describes the catalytic trimerization of monomeric 1,6-diisocyanatohexane (HDI) at 40° C. to give a clear transparent polyisocyanurate plastic free of isocyanate groups. For this purpose, however, 15% by weight of dibutyltin dimethoxide as trimerization catalyst is required. European Polymer Journal, Vol. 16, 831-833 (1980) describes the complete trimerization of monomeric HDI to give a polyisocyanurate at a temperature of 140° C. using 6% by weight of tributyltin oxide as catalyst.
Journal of Polymer Science Part A: Polymer Chemistry 2013, 51, 2631-2637 describes the production of optically transparent polyisocyanurate films based on monomeric HDI/MDI mixtures with sodium p-toluenesulphinate as catalyst. Using exclusively monomeric HDI as starting diisocyanate, however, it is not possible to obtain clear films with this catalyst, since the reaction mixture foams significantly because of the extreme exothermicity of the trimerization reaction. According to this publication, the synthesis of a pure HDI polyisocyanurate was possible only in a test tube on the mmol scale in organic solution after complex workup.
U.S. Pat. No. 3,211,703 describes solid crosslinked polymers consisting of at least ten successive isocyanurate structures joined via divalent aliphatic groups, preferably hexamethylene chains. In the specific working examples of this patent specification, however, exclusively copolymers of HDI with styrene oxide are described.
JP 2001-098042 describes polyisocyanurates proceeding from monomeric cycloaliphatic bis(isocyanatomethyl)norbornane (NBDI) which have an isocyanate group content of max. 13% in the end product and are prepared using a catalyst system consisting of potassium fluoride and a complexing agent containing ethylene oxide groups.
The thesis by Theo Flipsen: “Design, synthesis and properties of new materials based on densely crosslinked polymers for polymer optical fiber and amplifier applications”, Rijksuniversiteit Groningen, 2000 describes the trimerization of monomeric HDI with a neodymium/crown ether complex as catalyst. The polyisocyanurate obtained, which is said to have good optical, thermal and mechanical properties, was studied in the context of the thesis for its suitability for optical applications, especially as polymeric optical fibres.
However, processes known from the prior art for production of polyisocyanurate plastics from monomeric aliphatic diisocyanates have the fundamental disadvantage that a considerable shrinkage in volume occurs in the course of a trimerization reaction, which can present problems particularly in the case of casting of bodies of defined geometry. Moreover, it is a common factor in the production processes for polyisocyanurate plastics which proceed from the monomeric diisocyanates and are known from the prior art that they are very time-consuming and take place in closed systems under complex temperature control.
WO 2015/166983 discloses the use of isocyanurate polymers for encapsulating LEDs. It is explicitly disclosed that only those isocyanurate polymers which contain allophanate groups have the required technical properties.
U.S. Pat. No. 6,133,397 only discloses coatings made by trimerizing oligomeric polyisocyanates. It does not disclose the production of solid bodies.
The problem addressed by the present invention was therefore that of providing novel polyisocyanurate plastics having high thermal stability, which can be produced with considerably lower volume contraction and are thus also suitable particularly for production of bodies with defined geometry.